1. Field of the Invention
The present invention relates to an inflation film of an ultra-high-molecular-weight polyethylene. The present invention also relates to a method and apparatus for producing an inflation film from an ultra-high-molecular-weight polyethylene.
2. Description of the Related Art
Ultra-high-molecular-weight polyethylene is increasingly used as a so-called engineering plastic because of its excellent impact resistance, abrasion resistance, chemical resistance, tensile strength, and other properties, when compared with conventional polyethylene. However, ultra-high-molecular-weight polyethylene has an extremely high melt viscosity and poor flowability when compared with conventional polyethylene, and thus it is very difficult to mold such a plastic by conventional extrusion molding (e.g., inflation molding) and injection molding. Accordingly, ultra-high-molecular-weight polyethylene is generally molded by compression molding; with the exception of rods and the like, which are partly extrusion molded at a very low extrusion speed.
It has been proposed in, for example, Japanese Examined Patent Publication (Kokoku) No. 48-11576, as a method for producing a film from an ultra-high-molecular-weight polyethylene that, after the ultra-high-molecular-weight polyethylene powder is sintered, the sintered powder be heated between two flat belts at a temperature higher than the melting point of the polyethylene, under pressure and followed by cooling. Alternatively, it has been proposed in, for example, Japanese Unexamined Patent Publication (Kokai) No. 53-45376 that a sintered ultra-high-molecular-weight polyethylene film be orientated at a temperature of the second-order transition temperature or more to a temperature of less than the melting point thereof by means of press rolls. Although the former method can provide films having good appearance, the production of thinner films is difficult because the melt viscosity of ultra-high-molecular-weight polyethylene is extremely large. Furthermore, the latter method has disadvantages in that, since the viscosity of ultra-high-molecular-weight polyethylene becomes larger at a temperature of less than the melting point, a desirable film cannot be obtained even when the polyethylene is rolled by a compression roller.
Although it seems obvious that, as a method for producing a thin film, the ultra-high-molecular-weight polyethylene can be stretched, the ultra-high-molecular-weight polyethylene alone cannot be stretched at all because the molecular weight thereof is very large. Also, although it appears obvious to heat the polyethylene to the melting point thereof or more during the stretching, in general, this increases the stickness of the crystalline polyolefin and decreases the melt viscosity thereof, upon heating to the melting point thereof or more, so that the rolling becomes difficult. Accordingly, as pointed out in the above-mentioned Japanese Unexamined Patent Publication (Kokai) No. 53-45376, the desired film having a good appearance cannot be obtained at the melting point thereof or more because the molten resin adheres to the compression roller.
Furthermore, the present inventors have proposed in Japanese Unexamined Patent Publication (Kokai) No. 59-227420, as an improved method for producing a film of an ultra-high-molecular-weight polyethylene that the ultra-high-molecular-weight polyethylene is mixed with a large amount of a plasticizer, followed by extruding the mixture to produce a biaxially oriented film. Although the resultant biaxially oriented film has extremely excellent mechanical strength such as tensile strength, impact strength, etc., the permeability thereof to water vapor is larger than that of conventional polyethylene film. Thus, the application in usage thereof is naturally limited.